In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners which contain pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor's surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor's surface. A set of fuser rolls or belts, under heat, melts and fixes the toner in the paper subsequent to transfer, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and ongoing series of steps in which the photoconductor's surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor's surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to bond with the fibers of the paper, thereby fixing the image onto the paper or recording medium. While this is an effective way of fixing the toner image on the paper's surface, it carries with it some problems. Specifically, the simplest approach to fusing the toner is to apply a constant level of heat to the surface of the printing medium. Usually a closed loop control system is used to regulate this level of heat by controlling the temperature of the fuser hot roller or belt. Typically, a thermistor is used to sense the temperature of the fuser hot roller or the heater which heats the fuser belt. This can cause a problem when print media of various widths, such as labels, notepaper or envelopes, is fed into the printer, particularly a printer which utilizes a belt-type fuser and is reference edge fed. As used herein, "reference edge fed" means that one side of the media is always pressed against a reference surface in the printer. It is important to be able to feed media of various lengths and widths without incurring damage to the fuser. When feeding narrow print media, such as labels, notepaper or envelopes, the non-media side of the fuser will lose the thermal mass and heat-sinking effect of the media as it passes through the fuser nip, while the media side of the fuser will have the media to absorb some of the heat generated. This creates a non-uniform temperature profile along the axis of the heater since the thermistor will be controlling the temperature from a position on the heater covered by the media. As the media gets longer, heavier, and narrower, the difference in temperature between the media and non-media sides of the fuser increases significantly.
In hot-roller fuser mechanisms, the thermal mass of the hollow aluminum fuser roller provides a sufficient conduction path for the excess energy to flow from the non-media to the media side of the roller, thus avoiding damagingly high temperatures in the fuser. However, in belt fusers, the heating system has a very low thermal mass that does not create a good conduction path for the excess energy. Instead, the excess energy is driven into the fuser belt, heater housing and back-up roller, which cannot safely handle the damaging effects of the high temperatures. This can cause damage to the printer and the pages being printed. Given business demands, which require that belt fusers be able to feed all widths of print media safely, there is a need to be able to control fuser temperatures in the presence of narrow media. The present invention addresses this need in a very simple, inexpensive and effective manner.
The issue of controlling heat in the fuser when narrower print media is utilized has been addressed in the prior art. However, these approaches do not utilize the straightforward approach defined in the present invention.
U.S. Pat. No. 5,289,247, Takano, et al., issued Feb. 22, 1994, addresses the problem of fuser overheating in non-contact areas where narrower print media is fed into the printer. In this approach, the circuitry in the printer includes preset fuser heater temperatures and preset intervals at which print media is fed into the printer. The problem of higher temperatures when narrower print media is used is addressed by either moving to lower preset feeding speeds for the narrower print media, lower preset fuser roller temperatures, or preset intervals during which no print media is moved through the fuser. In the course of this approach, the fuser heater is turned on and off during the printing process, but this is done to achieve and maintain the preset temperatures which are programmed into the printer circuitry.
U.S. Pat. No. 5,315,350, Hirobe, et al., issued May 24, 1994, describes printer circuitry developed to utilize electricity as efficiently as possible during the fusing process. The Hirobe, et al. invention does not deal with fuser overheating caused by narrow print media being fed through the fuser. In this procedure, the heater for the fixing roll is turned on and off in order to keep the fixing roll temperature within a predetermined range. By doing this, the appropriate fixing temperature can be achieved without requiring that large surges of electricity be fed into the printer. A conversion table is utilized in the circuitry in order to determine how long the heater is to be left on to achieve these predetermined target temperatures, based on the current temperature of the roller.
U.S. Pat. No. 5,621,511, Nakayama, issued Apr. 15, 1997, relates to a procedure for adjusting the temperature of the fixing roller in a fuser without requiring that the copying time be extended. In this procedure, the power is adjusted on and off to maintain the fuser temperature within a fixed range, while the fuser mechanism adjusts the sheet feeder interval based on the temperature of the fixing device and the number of remaining sheets to be printed.
U.S. Pat. No. 5,669,039, Ohtsuka, et al., issued Sep. 16, 1997, describes a procedure for maintaining a uniform fuser belt temperature when media of different widths are fed into the fuser mechanism. This is accomplished by varying the level of electric power to the heater and the feed interval of media into the fuser.
It has now been found that by turning down or turning off the power to the fuser heater when a piece of narrow gauge printing media has exited the fuser nip, the problem of fuser belt overheating caused by such media can be overcome. This approach can be further enhanced by incorporating into the fuser a mechanism which periodically measures the temperature of the heater, when the heater is turned down or turned off; determining the amount of time it will take for the next piece of printing media to enter the fuser nip; and then reactivating power to the heater based on the amount of time it will take the heater to move from its current temperature to its operational temperature in view of the amount of time remaining before the next item enters the fuser nip.